U.S. patent number 5,684,396 [Application Number 08/611,352] was granted by the patent office on 1997-11-04 for localizing magnetic dipoles using spatial and temporal processing of magnetometer data.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Stanley O. Aks, Kirk Kohnen.
United States Patent |
5,684,396 |
Aks , et al. |
November 4, 1997 |
**Please see images for:
( Certificate of Correction ) ** |
Localizing magnetic dipoles using spatial and temporal processing
of magnetometer data
Abstract
Processing methods and apparatus that process magnetometer data
derived from an array of magnetometer sensors and outputs both the
position and velocity of a magnetic dipole. In the method and
apparatus, (a) a set of actual magnetic field measurements of a
magnetic dipole is collected using the array of magnetic sensors.
Then (b), a trajectory for the magnetic dipole is hypothesized.
Then (c), a set of estimated magnetic field measurements is
determined that would be formed by a magnetic dipole moving along
the hypothesized trajectory. Then (d), the actual magnetic field
measurements are compared with the estimated magnetic field
measurements. Then (e), based on the comparison, a new trajectory
for the magnetic dipole is hypothesized. Steps (c) through (e) are
repeated until agreement between the actual magnetic field
measurements and the estimated magnetic field measurements is
deemed sufficiently close. The trajectory is displayed for viewing
on a display.
Inventors: |
Aks; Stanley O. (Cerritos,
CA), Kohnen; Kirk (Fullerton, CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
24448690 |
Appl.
No.: |
08/611,352 |
Filed: |
March 5, 1996 |
Current U.S.
Class: |
324/207.13;
324/244; 324/207.26; 324/207.14; 702/150 |
Current CPC
Class: |
G01V
3/081 (20130101) |
Current International
Class: |
G01V
3/08 (20060101); G01R 033/02 () |
Field of
Search: |
;324/207.13,207.14,207.11,207.22,207.26,244,247
;364/423.098,449.1,556,559,424.012 ;340/988,995 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Keiichi Mori, "Application of Weight Functions to the Magnetic
Localization of an Object", printed in the IEEE Transactions on
Magnetics 25 (1989) May, No. 3, New York, U.S., pp.
2726-2731..
|
Primary Examiner: O'Shea; Sandra L.
Assistant Examiner: Patidar; J. M.
Attorney, Agent or Firm: Lachman; M. E. Sales; M. W.
Denson-Low; W. K.
Claims
What is claimed is:
1. A method of detecting and localizing a magnetic dipole using an
array of spatially distributed magnetic sensors, said method
comprising the steps of:
a) collecting a set of actual magnetic field measurements of a
magnetic dipole using a plurality of magnetic sensors;
b) hypothesizing a trajectory for the magnetic dipole;
c) determining a set of estimated magnetic field measurements that
would be formed by a magnetic dipole moving along the hypothesized
trajectory;
d) comparing the actual magnetic field measurements with the
estimated magnetic field measurements;
e) based on the comparison, hypothesizing a new trajectory for the
magnetic dipole; and
f) repeating steps c) through e) until agreement between the actual
magnetic field measurements and the estimated magnetic field
measurements is deemed sufficiently close.
2. The method of claim 1 further comprising the step of displaying
the trajectory data.
3. The method of claim 1 wherein the step of repeating steps c)
through e) until agreement between the actual and estimated
magnetic field measurements is deemed sufficiently close comprises
the steps of:
correlating the measured magnetic field values with each of the
estimated magnetic field values for the array of sensors, by
multiplying the estimated magnetic field values with the measured
magnetic field values and summing the results over the array of
sensors; and
if one of the resulting correlations has a significantly larger
value than the others and if it is greater than a predetermined
threshold, declaring a detection for the location corresponding to
the calculated values that resulted in the larger correlation
value.
4. The method of claim 3 further comprising the step of displaying
the trajectory data.
5. A method of detecting and localizing a magnetic dipole using an
array of spatially distributed magnetic sensors, said method
comprising the steps of:
a) collecting a set of actual magnetic field measurements of a
magnetic dipole using a plurality of magnetic sensors;
b) filtering the actual magnetic field measurements using a
predetermined filter;
c) hypothesizing a trajectory for the magnetic dipole;
d) determining a set of estimated magnetic field measurements that
would be formed by a magnetic dipole moving along the hypothesized
trajectory;
e) filtering the estimated magnetic field measurements using the
same predetermined filter;
f) comparing the actual magnetic field measurements with the
estimated magnetic field measurements;
g) based on the comparison, hypothesizing a new trajectory for the
magnetic dipole; and
h) repeating steps d) through g) until agreement between the actual
magnetic field measurements and the estimated magnetic field
measurements is deemed sufficiently close.
6. The method of claim 5 further comprising the step of displaying
the trajectory data.
7. Apparatus for detecting and localizing a magnetic dipole
comprising:
an array of magnetic sensors;
processing means coupled to the array of magnetic sensors, for
storing an estimate of the magnetic field signature to be detected
by the array of sensors at each of a plurality of locations to
provide an array of estimated magnetic field signals that are
represented by fundamental magnetic dipole equations, for
collecting magnetic field signals indicative of the magnetic field
measured at each of the sensors in the presence of the magnetic
dipole that is to be detected to provide measured magnetic field
values, for removing spatial and temporal variations in the
magnetic field measured at each of the sensors by temporally and
spatially filtering the measured magnetic field signals, for
correlating the measured magnetic field values with each of the
estimated magnetic field values for the array of sensors by
multiplying the estimated magnetic field values with the measured
magnetic field values and summing the results over the array of
sensors, for identifying the trajectory of the dipole if one of the
correlations has a significantly larger value than the others and
if it is greater than a predetermined threshold, and if the
trajectory of the dipole corresponds to the location represented by
the measured magnetic field signals that produced in the
significantly larger value; and
display means for displaying the location and velocity of the
identified dipole.
Description
BACKGROUND
The present invention relates generally to magnetometer data
processing methods and apparatus for localizing magnetic dipoles,
and more particularly, to methods and apparatus that employs
spatial and temporal processing of magnetometer data to localize
magnetic dipoles and provide both the position and velocity of the
dipole.
Metal objects such as firearms, automobiles, ships, and submarines,
for example, have magnetic dipole moments that may be used to
detect them. Historically, magnetic field sensors have been used to
detect (but not locate) such objects. Magnetic dipole detectors
developed by the assignee of the present invention have been used
to localize objects in two ways. One implementation uses a single
sensor and data is sensed over a period of time to localize the
magnetic dipole. The other implementation uses an array of sensors,
and a local time average of each sensor output is determined. This
data is processed to determine the location (but not the velocity)
of the dipole. This second technique makes an implicit assumption
that the dipole of interest is relatively stationary while the
measurements are being taken.
Prior art relating to the present invention is disclosed in U.S.
Pat. No. 5,239,474 entitled "Dipole Moment Detection and
Localization" assigned to the assignee of the present invention.
This patent discloses a dipole moment detection and localization
algorithm that is used to process magnetometer data to localize
magnetic dipoles. However, the algorithm described this patent does
not provide an indication of the velocity of the dipole as a direct
output thereof. The present invention provides for a processing
method or algorithm that improves upon the teachings of this
patent.
Accordingly, it is an objective of the present invention to provide
for methods and apparatus that employs spatial and temporal
processing of magnetometer data to localize magnetic dipoles and
provides both the position and velocity of the dipole.
SUMMARY OF THE INVENTION
To meet the above and other objectives, the present invention
provides for processing methods and apparatus that processes
magnetometer data derived from an array of magnetometer sensors and
outputs both the position and the velocity of a magnetic dipole as
a direct output thereof. A physically distributed array of
magnetometer sensors is used to sense the magnetic signature of a
magnetic dipole. A set of magnetometer readings derived from the
physically distributed array of magnetometer sensors is sampled
over a predetermined period of time. The set of magnetometer
readings is processed to estimate the trajectory (including both
the location and the velocity) of the magnetic dipole. The present
invention processes data from the magnetic sensors, taking into
account changes in the measurements due to the motion of the
magnetic dipole. Consequently, the immunity to noise of the present
invention is greater than that of previously developed algorithms
or apparatus.
In the present invention, (a) a set of actual magnetic field
measurements of a magnetic dipole is collected using a plurality of
magnetic sensors. Then (b), a trajectory for the magnetic dipole is
hypothesized. Then (c), a set of estimated magnetic field
measurements is determined that would be formed by a magnetic
dipole moving along the hypothesized trajectory. Then (d), the
actual magnetic field measurements are compared with the estimated
magnetic field measurements. Then (e), based on the comparison, a
new trajectory for the magnetic dipole is hypothesized. Steps (c)
through (e) are repeated until agreement between the actual
magnetic field measurements and the estimated magnetic field
measurements is deemed sufficiently close.
The processing method and apparatus of the present invention
provides for a substantial improvement over the processing
technique described in U.S. Pat. No. 5,239,474. The present
processing method and apparatus provides an estimate of both the
position and the velocity of the magnetic dipole that is tracked.
In many applications, such as when tracking vehicles, for example,
knowing the velocity of the magnetic dipole is of primary
importance. Additionally, the present processing method and
apparatus has better noise immunity than the prior art invention
described in U.S. Pat. No. 5,239,474 applied to a single time
slice, or to a set of time-averaged data.
The spatial and temporal processing methods and apparatus of the
present invention may be used with any system intended to passively
detect, locate, and classify objects using their magnetic fields.
The present processing method or algorithm may be employed in
non-acoustic anti-submarine surveillance and warfare systems,
airport ground-traffic control systems, highway traffic monitoring
systems, and personal weapon detection systems, and may provide for
clandestine monitoring of military activity behind enemy lines, for
example.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be
more readily understood with reference to the following detailed
description taken in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural elements,
and in which:
FIG. 1 shows magnetic dipole detection apparatus in accordance with
the principles of the present invention;
FIG. 2 shows a flow diagram illustrating a processing method or
algorithm in accordance with the present invention employed in the
apparatus of FIG. 1; and
FIG. 3 shows a track that results from using the present
invention.
DETAILED DESCRIPTION
Referring to the drawing figures, FIG. 1 shows magnetic dipole
detection apparatus 10 in accordance with the principles of the
present invention. The magnetic dipole detection apparatus 10
comprises an arbitrarily positioned array of magnetic sensors 11
that is coupled to processing apparatus 16 that implements
processing methods 20 in accordance with the present invention. The
sensors 11 of the array of magnetic sensors 11 may be coupled to
the processing apparatus 16 by means of hard wire coupling, an RF
data link, a microwave data link, or other appropriate means. A
magnetic dipole 13 (or target 13) is located within the detection
range of the array of magnetic sensors 11. A plurality of arrows
representing magnetic vectors 14 are shown extending from each
sensor 11 a of the array of magnetic sensors 11 toward the location
of the moving dipole 13. The readings from each sensor 11 comprise
a set of data that are indicative of the location of the dipole 13
during the period of time during which the sensor data is gathered
for analysis. The processing method 20 or algorithm described
herein provides a technique whereby the motion of the target is
approximated by constant velocity motion for each subset of
collected data and then the actual positions of the dipole 13 that
correspond to this motion are estimated. The sensor data from the
array of magnetic sensors 11 is processed in the processing
apparatus 16 to generate position and velocity information
regarding the dipole 13 and this data is subsequently displayed for
viewing on a display 17.
FIG. 2 shows a flow diagram illustrating a processing method 20 or
algorithm in accordance with the present invention employed in the
apparatus 10 of FIG. 1. The processing method 20 is implemented in
the processing apparatus 16 and comprises the following steps. The
first step (a) involves collecting 21 a set of actual magnetic
field measurements of the magnetic dipole 13 using a plurality of
magnetic sensors 11. Optionally, the actual magnetic field
measurements may be filtered by a predetermined filter, as shown in
step 28. In the next step (b), a trajectory for the magnetic dipole
13 is hypothesized 22. In the next step (c), a set of estimated
magnetic field measurements is determined 23 that would be formed
by a magnetic dipole moving along the hypothesized trajectory. In
the event that the actual magnetic field measurements have been
filtered by a predetermined filter, the estimated magnetic field
measurements are also filtered by the predetermined filter as shown
in step 29. Then in the next step (d) the actual magnetic field
measurements (or filtered actual measurements) are compared 24 with
the estimated magnetic field measurements (or filtered estimated
measurements). Then in step (e), based on the comparison, a new
trajectory for the magnetic dipole is hypothesized 25. Steps c)
through e) are repeated 26 until agreement between the actual
magnetic field measurements and the estimated magnetic field
measurements is deemed sufficiently close. The trajectory is
displayed 27 for viewing on a display 17.
The spatial and temporal processing performed by the present
invention is mathematically described as follows. The magnetic
field at a point in space due to the presence of the magnetic
dipole 13 is given by the basic equation:
where: B is the magnetic field vector, given by:
m is the dipole moment vector, given by:
r is the position vector, given by:
r(t) is the position in the magnetic field with respect to the
position of the dipole moment, given by:
r.sub.s is the position at which the magnetic field equals B, and
r.sub.d (t) is the position of dipole moment at time t.
After substituting the vector components and simplifying, the basic
equation becomes: ##EQU1## where:
The position matrix may be defined as: ##EQU2## Then, by
substitution:
The position matrix, R, is a function of the relative positions of
the magnetic dipole 13 and the point in space where the magnetic
field equals B.
Assume that the array of magnetic sensors 11 is arbitrarily
arranged in three-dimensional space such as is shown in FIG. 1.
Each sensor of the array 11 measures the three components of the
local magnetic field. The sensors of the array 11 are oriented so
that their respective axes are parallel to each other. Further
assume that the dipole 13 is moving along a trajectory in
three-dimensional space given by:
Define the measurements of the magnetic field at the array of
sensors 11 by a composite vector ##EQU3## where N is the number of
sensor data channels, and T is number of times the sensors 11 are
sampled.
The position matrix for the array 11 may be defined by the
composite matrix: ##EQU4##
Then for the array, the expansion of equation 9 becomes:
If B.sub.R is a set of measurements representing the field of a
single magnetic dipole 13 moving along an assumed trajectory
r.sub.d (t)=(x.sub.0 +v.sub.x t)i +(y.sub.0 +v.sub.y t)j+(z.sub.0
+v.sub.z t)k, solving equation 13 for m provides an estimate of the
magnetic dipole vector. It is to be understood that the trajectory
of the dipole 13 is not limited to this form, but may be any
continuous function. At this point, assume that other significant
magnetic sources, such as the earth's magnetic field and local
geomagnetic distortions, have been subtracted from the
measurements. Define the pseudo-inverse of the array position
matrix as:
Then the estimated dipole vector is:
The correlation coefficient will now be derived. The goodness of
the estimated dipole vector of the magnetic dipole 13 is evaluated
as the correlation coefficient between actual sensor measurements
and ideal sensor measurements derived from the estimated dipole
vector. Define the set of ideal measurements as
By subtracting the mean from the sets of actual and ideal
measurements, zero-mean vectors are obtained that are given by:
##EQU5## Then, the correlation coefficient is: ##EQU6##
The estimate of measurements will now be discussed. B.sub.I is an
estimate of the realizable measurements at the sensors that best
fits the actual sensor measurements B.sub.R. If the expression for
m.sub.est is substituted into the equation for B.sub.I, a direct
estimate of the ideal measurements is obtained:
Define the measurement estimation matrix:
which, in expanded form is:
Then, substituting equation 21 into equation 20 yields:
S is a function of sensor positions and hypothesized dipole
position. It has the properties of being symmetrical and optimal in
that:
Equation 24 shows that the estimator S, when applied to an ideal
set of measurements, B.sub.I, reproduces the set of ideal
measurements as an optimal estimate.
To estimate the trajectory of the dipole 13, the value of: ##EQU7##
is maximized by varying the values of six parameters that define
the trajectory of the dipole 13 which are given by:
In the case of multiple dipoles 13, the linear form of the dipole
estimator equation simplifies the simultaneous processing of data
derived from the multiple dipoles 13. Assume two magnetic dipoles
13, m.sub.1 and m.sub.2, at two different positions characterized
by R.sub.1 and R.sub.2. Assume that the same array of sensors 11 is
used to measure both m.sub.1 and m.sub.2. Because equation 9 is
linear, and because magnetic fields may be linearly summed, the
effects of the two dipoles 13 on the sensor measurements may be
linearly summed:
which may be simplified to: ##EQU8##
Equation 26 shows that multiple dipoles 13 at hypothesized
locations may be estimated simultaneously from one set of
measurements. In the general case, define: ##EQU9## where M is the
numbers of dipoles 13, and also define:
Then equation 29 becomes:
This leads to the simultaneous estimation of the locations of M
dipole sources:
where R.sub.T is the pseudo-inverse of R.sub.T as in equation
14.
As was stated above, the present method 20 also envisions filtering
the actual and estimated magnetic filed measurements, which filters
the measurements against noise. In the filtering embodiment, the
collected data is multiplied by a square matrix that embodies a
filter that reduces the effect of noise. The magnetic field vectors
that result from the hypothesized dipole trajectory are multiplied
by the same square matrix. The matrix is selected to improve the
performance of the method against the noise.
The following equations describe the present method 20 with
filtering:
FIG. 3 shows a track that results from processing magnetometer data
using the present method 20. Using an array of eight 3-axis
magnetometers, the spatial temporal processing method 20 of the
present invention provides a three-dimensional track of a dipole
13, such as for a large cruise ship for the example shown in FIG.
1.
Thus, improved methods and apparatus that employ spatial and
temporal processing of magnetometer data to localize magnetic
dipoles and output trajectory data regarding the dipole have been
disclosed. It is to be understood that the described embodiments
are merely illustrative of some of the many specific embodiments
which represent applications of the principles of the present
invention. Clearly, numerous and varied other arrangements may be
readily devised by those skilled in the art without departing from
the scope of the invention.
* * * * *